1. Field of the Invention
The present invention relates to a device measuring dispersion on an optical transmission line, and more particularly, to a device monitoring a dispersion value of an optical transmission line and optimally compensating for dispersion.
2. Description of the Related Art
Currently, a 10-Gbps optical transmission system starts to be put into use. With a sharp increase in network use in recent years, the demand for further increasing a network capacity has been growing. Since dispersion compensation must be made with high accuracy at a transmission speed of 10 Gbps or faster, it is essential to accurately measure the dispersion value of a transmission line.
In an optical transmission system of a transmission speed of 10 Gbps or faster, its wavelength dispersion tolerance is very small. For example, the wavelength dispersion tolerance of a 40-Gbps NRZ system is 100 ps/nm or smaller. In contrast, for a ground optical transmission system, a relay section is not always uniform. A system using a 1.3 xcexcm zero-dispersion SMF (Single Mode Fiber) of approximately 17 ps/nm/km exceeds the wavelength dispersion tolerance, even if a relay section varies only by several kilometers.
However, in an optical fiber network possessed by a carrier, most of the distances and the wavelength dispersion values of relay sections are not accurately grasped. Furthermore, since a wavelength dispersion value changes with time due to the temperature of an optical fiber or the stress applied to an optical fiber, etc., the dispersion compensation amount for each relay section must be suitably set while strictly measuring the wavelength dispersion amount not only at the start of system use but also in system use. For example, if a temperature change of 100 degrees centigrade occurs on a DSF (Dispersion Shifter Fiber) transmission line of 500 kilometers, a wavelength dispersion change amount results in approximately 105 ps/nm, which is nearly equivalent to the wavelength dispersion tolerance of an NRZ signal.
(wavelength dispersion change amount)=(temperature dependency of a zero-dispersion wavelength)xc3x97(temperature change amount in a transmission line)xc3x97(dispersion slope of a transmission line)xc3x97(transmission distance)=0.03 (nm/xc2x0 C.)xc3x97100 (xc2x0 C.)xc3x970.07 (ps/nm2/km)xc3x97500 (km)=105 ps/nm 
where the dispersion slope of a transmission line is a differentiated value (ps/nm2/km) of a dispersion amount, which will be described later.
Accordingly, a system automatically measuring a dispersion amount is essential not only for an SMF transmission line but also for a system using a 1.55 xcexcm zero-dispersion DSF or an NZ-DSF transmission line.
As a currently used wavelength dispersion monitoring method, the following two methods can be cited.
1. twin-pulse method
2. optical phase comparison method
FIG. 1 shows the outline of the configuration of a wavelength dispersion measuring device using the twin-pulse method.
The twin-pulse method is a method obtaining a wavelength dispersion amount (group delay) by using two optical pulse signals having different wavelengths, and by measuring the delay difference between the two pulses after being transmitted over a fiber to be measured. In this case, two LDs producing different wavelengths, and their driving units are required.
First of all, an electric signal pulse is generated from a pulse generator 10, and at the same time, a trigger signal for starting measurement is transmitted to a group delay measuring instrument. The electric pulse transmitted from the pulse generator 10 is input to two driving units 11-1 and 11-2, which are made to simultaneously output optical pulses to LDs 12-1 and 12-2 that respectively produce lights having wavelengths xcex1 and xcex2. Optical pulses produced by the LDs 12-1 and 12-2 are multiplexed by an optical multiplexer such as a half mirror 13, a coupler, etc., and are propagated through an optical fiber transmission line 14. The two optical pulses that propagate through the optical fiber transmission line 14 are detected by a detector 15, and the detection result of the optical pulses is transmitted to the group delay measuring instrument 17. In the meantime, the trigger signal output from the pulse generator 10 is delayed in a delaying circuit 16 by an amount of time required to propagate the optical pulses through the optical fiber transmission line, and input to the group delay measuring unit 17 as a trigger signal for starting up the group delay measuring unit 17.
The group delay measuring unit 17 detects the difference between the arrival times of the two optical pulses detected by the detector 15, and calculates the group delay times of the optical pulses having the wavelengths xcex1 and xcex2.
FIGS. 3A and 3B show the states of optical pulses propagated with the twin-pulse method.
As shown in FIG. 3A, optical pulses having wavelengths xcex1 and xcex2 are simultaneously generated, multiplexed, and output. Since the optical pulses having the wavelengths xcex1 and xcex2 are simultaneously output at this time, the pulses are multiplexed into one and input to a transmission line as shown on the right side of FIG. 3A. However, a group delay is caused by the wavelength dispersion of the transmission line. Therefore, when the optical pulses having the wavelengths xcex1 and xcex2 are received on a receiving side, there is a reception time lag between the optical pulses as shown in FIG. 3B. Here, it is assumed that the group delay of the optical pulse having the wavelength xcex1 is larger than that of the optical pulse having the wavelength xcex2.
Accordingly, a group delay time can be calculated based on the difference between the arrival times of the two optical pulses having the wavelengths xcex1 and xcex2, and a wavelength dispersion amount can be calculated by using the difference between the wavelengths xcex1 and xcex2.
FIG. 2 shows the outline of the configuration of a wavelength dispersion measuring device using the optical phase comparison method.
The optical phase comparison method is a method obtaining a wavelength dispersion amount not by directly measuring a group delay time difference, but by acquiring a phase difference between modulated optical signals, which is caused by a group delay time difference.
First of all, an electric pulse is generated by a pulse generator 10. At the same time, a trigger signal for notifying a phase detector 18 of a measurement reference time of the propagation time of an optical pulse is transmitted by the pulse generator 10.
The electric pulse transmitted from the pulse generator 10 is input to a driving unit 11, and an optical pulse having a wavelength xcex is output from an LD 12. This optical pulse propagates through a transmission line 14, and is detected by a photodetector 15. The photodetector 15 inputs the signal indicating the detection of the optical pulse to a phase detector 18. The phase detector 18 measures the delay time of the arrival of the optical pulse with reference to the time at which the trigger signal is received from the pulse generator 101.
Then, the wavelength of the optical pulse transmitted from the LD 12 is changed, and the above described measurement is repeated in a similar manner. As a result, the propagation time of the optical pulse, which indicates the delay time when the optical pulse transmitted with the different wavelength is detected on a receiving side from the reference time, may differ. This propagation time difference is a group delay time difference. When the group delay time difference is obtained in this way, the wavelength dispersion of a transmission line is obtained from the wavelength difference and the group delay time difference.
FIG. 3C shows the state of optical pulses used in the optical phase comparison method.
If the input time of the trigger signal input from the pulse generator 10 to the phase detector 18 is used as a reference, there is almost no difference between the travel distances of the optical pulses having the wavelengths xcex1 and xcex2 at the reference time. Namely, the reference time is a time point immediately after the optical pulse is output from the LD 12, an instant when the optical pulse it output, etc. Accordingly, no influence of wavelength dispersion on the transmission line has been exerted yet. However, when the optical pulse having the wavelength xcex1 is detected by the photodetector 15, the optical pulse is in the state of being influenced by the wavelength dispersion on the transmission line after being propagated. Also the optical pulse having the wavelength xcex2 is in the same state when being detected by the optical detector 15.
Here, if the wavelength dispersion of the wavelength xcex1 is assumed to be larger than that of the wavelength xcex2, the speed at which the optical pulse having the wavelength xcex1 propagates through the transmission line is slower than that of the optical pulse having the wavelength xcex2. As a result, the amount of time required until the optical pulse having the wavelength xcex1 is detected after propagating through the transmission line 14 becomes longer. Accordingly, a group delay time caused by the wavelength dispersion of a transmission line can be measured by detecting the difference between the times when the optical pulses having the wavelengths xcex1 and xcex2 arrive at the photodetector 15. Then, the wavelength dispersion can be measured from the group delay time and the wavelength difference.
However, since the propagation time of a transmission line is included and the group delay time difference between two waves is obtained from a phase difference with these methods, the propagation delay time of a transmission line must be accurately obtained. Additionally, the number of components such as LDs, driving units, etc. are large. Furthermore, a high SNR is required to make a phase comparison on the order of GHz at a receiving end, leading to a difficulty in securing the optical SNR at the receiving end of a wavelength dispersion light to be monitored.
An object of the present invention is to provide a wavelength dispersion measuring device which has a small number of components, and causes no optical SNR problem.
A wavelength dispersion measuring device according to the present invention comprises: a modulating unit modulating a light output from a light source; a transmitting unit transmitting the modulated light to a transmission line as a wavelength dispersion measurement light; a light receiving unit receiving the modulated light which propagates through the transmission line; a filter unit extracting a sideband signal from the received modulated light; and a phase difference detecting unit detecting a phase difference between different frequency band components of the sideband signal, and is characterized in that the wavelength dispersion value of the transmission line is calculated from the phase difference.
A wavelength dispersion measuring method according to the present invention comprises the steps of: (a) modulating a light output from a light source; (b) transmitting the modulated light to a transmission line as a wavelength dispersion measurement light; (c) receiving the modulated light which propagates through the transmission line; (d) extracting a sideband signal from the received modulated light; and (e) detecting a phase difference between different frequency band components of the sideband signal, and is characterized in that the wavelength dispersion value of the transmission line is calculated from the phase difference.
According to the present invention, a wavelength dispersion value is measured with the wavelength difference between side lobes of a wavelength dispersion measurement light. Accordingly, there is no need to measure the propagation delay time of a transmission line, leading to simplification of the circuitry configuration of the device, and a reduction in the number of components. Additionally, if a sideband signal is extracted by using heterodyne detection, the frequency band can be dropped from an optical to an electric frequency, and the phase difference between sideband signals can be accurately detected by measuring the electric signal, etc. Consequently, a noise amount included at the time of the detection of the phase difference can be reduced, thereby improving the optical SNR.